1. Field of the Invention
The present invention relates to an amplifier circuit.
2. Description of the Background Art
Cascode stages are known from prior art, in particular having two bipolar transistors, for example two npn transistors or two pnp transistors, wherein the collector of the first of the two transistors is connected to the emitter of the second of the two transistors.
For the most part, cascode stages are operated in a standard configuration that is described below and illustrated in FIG. 2. The emitter of the first transistor Q1′ is connected to ground (common-emitter circuit), while an input signal is present at the base. The base of the second transistor Q2′ is connected to a fixed DC voltage UB (common-base circuit), while the collector is connected to the supply voltage VCC through a load resistor RL′. Standard configuration cascode circuits frequently serve as a substitute for single transistors. One reason for this is the excellent consistency of the output current over output voltage for fixed control current.
Another conventional circuit topology for replacing a single transistor by a cascode circuit is a feedback circuit shown in FIG. 3. Here, the feedback branch, which is formed of the resistors R1″ and R2″, is connected to the load resistor RL″ and also to the base of the first transistor Q1″, so that the output voltage is fed back to the input of the first transistor Q1″ as well as to the input of the second transistor Q2″.